The present invention relates generally to diagnostic imaging and, more particularly, to a method and system of MR imaging with enhanced flow contrast. The present invention is well-adapted to reconstructing phase-contrast MR images with the suppression of non-zero spatially varying background phase occurring in phase-difference images primarily attributable to eddy currents resulting from the application of magnetic gradients to encode flowing nuclei to static tissue. Moreover, the present invention provides an effective technique to quantitatively assess flow velocities in vivo more accurately.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
Phase contrast (PC) imaging is an MR acquisition and reconstruction technique that uses flow-encoding gradients with a non-zero first moment to encode spins with a phase proportional to their velocity,φv=2πγM1v, where v is the spin velocity, M1 is the gradient first-moment, and γ is the gyromagnetic ratio for protons. The gradient first moment can be calculated from the gradient strength, G, as
      M    1    =            ∫      0      TE        ⁢                  G        ⁡                  (          t          )                    ⁢      t      ⁢                          ⁢                        ⅆ          t                .            As such, in a reconstructed PC image, spins flowing at faster velocities may be distinguished from slower flowing spins and from stationary spins. In this regard, PC imaging is commonly used to quantitatively assess flow velocities in vivo. PC imaging is also used to determine the shape of a corpus having flow therethrough.
In MR imaging there are many different sources of phase including differences between the transmit and receive phase, data acquisition window alignment, imperfect slice-selection, filtering, and eddy currents. Typically in PC, to remove these other sources of phase, two separate acquisitions with different first moments and the same zeroth moment (area) are used to remove the phase contribution.
There are two different reconstruction methods for PC, complex-difference (CD) reconstruction and phase-difference (PD) reconstruction. In CD reconstruction, the magnitude of the complex signal difference is calculated from each of the two flow-encoded acquisitions. CD images have zero signal in regions of stationary spins and a non-zero signal proportional to the transverse magnetization and the sine of the velocity-induced phase shift. In PD reconstruction, the difference of the phase from each acquisition is calculated, asΔφv=arg(Z1Z2*),where Z1 and Z2 represent the first and second flow-encoded acquisitions, * refers to the complex conjugate, and arg is the phase angle. This is equivalent toΔφv=2πγΔM1v, where ΔM1 is the difference in the first moments between the two acquisitions. The velocity-encoding value or venc is defined as the velocity at which spins will be imparted with a phase angle of π radians, orvenc=1/2γΔM1.
Notwithstanding its robustness, phase attributable to eddy currents induced by the flow-encoding gradients themselves is not removed with PD correction. Since these flow-encoding gradients differ between the two separate acquisitions, the eddy currents do not cancel one another when the two separate acquisitions are combined. As a result, the residual eddy currents may manifest themselves as non-zero spatially varying background phase in the PD images. This non-zero background phase is in addition to the phase accumulated by moving spins and can affect the precision of quantitative assessments.
Conventionally, background phase in a PD image is estimated by performing a first-order (or higher) polynomial fit to the image phase. This fit can then be subtracted from the PD image to assess or otherwise remove the background phase. However, this fitting does not distinguish between phase due to actual flowing spins and residual background phase. As such, the determination of the background phase can be effectively over- or under-calculated. In this regard, the presence of phase due to flowing spins can perturb the results of any assessment of the background phase. As a result, quantitative assessment of flow velocities can be affected by the eddy current-induced phase. Accordingly, background phase correction is not usually carried out for quantitative flow protocols.
One proposed solution to correct background phase influences on a PD image is to image a phantom with the same scan parameters used to image the subject. From the phantom scan, background phase is determined and is used to remove the background phase from the PD. While reasonably effective, separately scanning a phantom increases scan time, decreases throughput, and requires additional scanner setup and operator involvement.
Therefore, it would be desirable to have a system and method capable of discriminating between the phase of inflowing spins and the phase of background spins such that background phase correction may be effectively applied for quantitative assessments of flow velocities in vivo.